It is previously known to use resolvers as position transducers in industrial robots. To achieve the desired high accuracy of the position value obtained with the aid of such a resolver, the resolver is usually arranged such that the operating range of the axis corresponds to a plurality of revolutions of the resolver. This can be achieved, for example, by mounting the resolver on the shaft of a motor which drives the robot axis through a gear. However, this solution has the drawback that the resolver signal does not give an unambiguous definition of the position of the axis, so that the equipment must be supplemented with electronic means in the form of counters, memories, and the like, which continuously check which resolver revolution the axis is at within the moment. These electronic means must be reset when putting a robot into service and thereafter after each voltage drop out or other switch-off of the voltage supply of the robot. These so-called synchronization operations require time and manual labor.
Therefore, it is desired to provide so-called absolute-position measuring transducer systems, that is systems which provide an unambiguous, and accurate, determination of the position of a robot axis without requiring any synchronization operations. It has been proposed to design such a system with two resolvers for each robot axis. One resolver is arranged so as to rotate a plurality of revolutions when the robot axis moves between the limits of its operating range, and it then makes possible an accurate determination of the position. The other resolver is arranged so as to rotate less than one revolution when the robot axis moves between the limits of its operating range, and with the aid of this resolver an unambiguous determination may be obtained as to within which revolution the first resolver is situated. Thus, by combining the output signals of the two resolvers, an unambiguous determination of the position of the robot axis may be obtained. However, this solution requires two resolvers per robot axis and is therefore complicated and expensive.
Prior art position transducer equipment using resolvers have been connected to a control system common to the robot axes, which system then comprises supply and sensing members for the resolvers. Since each resolver has three windings, an extensive cable arrangement between the resolvers and the control system is then required. Such a cable arrangement will be expensive and bulky. This is particularly the case with absolute position-measuring transducer systems having two resolvers per robot axis.
From EP-A-177 901 it is previously known to provide an absolute position-measuring transducer system for an industrial robot by arranging for each robot axis two transducers, such as a resolver and a pulse transducer with associated circuits for sensing the direction of movement and counting the total number of revolutions of the axis. In normal operation, the number of whole revolutions for each axis, obtained from the sensing circuits of the pulse transducer, is combined in a calculation circuit with the angular position within each revolution, which is obtained from the resolver of the same axis. In case of drop out of the supply voltage, the pulse transducer with associated reading circuits is supplied by a battery and stores a value which correctly indicates the number of whole revolutions of the axis independently of movements of the robot during the voltage drop out. The pulse transducer may be fed from an oscillator with a low on/off ratio for reduction of the power consumption during the battery supply interval. Thus, in this known system, the resolver with its high current consumption is not used during the battery feed interval, but the resolver is supplemented with another type of position transducer, a pulse transducer, which itself has a low power consumption. In the system known from EP-A 177 901, there are thus required for each axis a resolver and a pulse transducer with associated sensing and storage circuits. A typical industrial robot has six axes, and the absolute-measuring function is thus obtained at the cost of a considerable complication and increase in price of the robot.